DC motors worksheet with answers

Free printable DC motors worksheet: 6 problems from easy to exam level, with an answer key. Print it or save it as a PDF.

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1.Easy

A DC motor turns at 1450 rpm, and its back-EMF at that speed is 105.2 V. Find the motor constant k in V·s/rad.

E = 105.2 Vn = 1450 rpm

k = V·s/rad

2.Easy

A DC motor with an armature resistance of 1.2 Ω is switched directly onto a 240 V supply. Its normal full-load current is 20 A. Find the armature current I_start at the moment of starting.

V = 240 VR_a = 1.2 Ω

I_start = A

3.Medium

A DC motor with k = 1.5 N·m/A and R_a = 1.2 Ω is connected to 240 V. Find its stall torque T_stall, the torque at zero speed.

V = 240 Vk = 1.5 N·m/AR_a = 1.2 Ω

T_stall = N·m

4.Medium

A DC motor with k = 1 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 240 Vk = 1 V·s/rad

n₀ = rpm

5.Hard

A DC chopper feeds a motor (k = 0.08 V·s/rad, R_a = 1 Ω) from a 24 V DC bus. The average armature voltage is D·24 V. Find the duty cycle D that runs the motor at 300 rpm with a load of 0.16 N·m.

V_dc = 24 Vk = 0.08 V·s/radR_a = 1 ΩT = 0.16 N·mn = 300 rpm

D = %

6.Hard

A DC motor (k = 0.6 V·s/rad, R_a = 0.4 Ω) on a fixed 120 V supply drives a load of 12 N·m. Find the series resistance R_s that slows it down to 600 rpm.

V = 120 Vk = 0.6 V·s/radR_a = 0.4 ΩT = 12 N·mn = 600 rpm

R_s = Ω

Answer key

1. k = 0.6928 V·s/rad
2. I_start = 200 A
3. T_stall = 300 N·m
4. n₀ = 2292 rpm
5. D = 18.81 %
6. R_s = 3.715 Ω

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